The present invention relates to a semiconductor laser element utilizing for an apparatus for recording and reproducing an optical information.
In recent years, semiconductor laser elements are actively investigated and developed as a key device of a system for recording and reproducing an optical information.
In the course of such development, a digital versatile disc (DVD) has been made practicable, in response to requests for a higher recording density and a greater storage capacity than those of a compact disc (CD). In addition, for an apparatus for reproducing of this DVD, it is requested that the apparatus can also reproduce CD or read the data of CD from the necessity of an identical optical information system as well as of inheritance and utilization of the forepassed data.
A red semiconductor laser having a wavelength band of 635-650 nm is utilized in a pickup for reproducing DVD and, on the other hand, an infrared semiconductor laser having a wavelength band of 780 nm is utilized in a pickup for reproducing CD. Most CD (for example, music CD, or CD-ROM) can be reproduced or the data thereof can be read even by the semiconductor laser having a wavelength band of 635-650 nm since, in general, when a wavelength of the laser becomes short, then the diameter of the read out beam becomes small. But, for a compact disc-recordable (CD-R), reproduction or readout of the data is impossible by such the red semiconductor laser, since the reflectance of the recording disc thereof is low for the light having a wavelength of 635-650 nm.
In order to make it possible to reproduce all of DVD, CD and CD-R or to read the data thereof by one apparatus, a pickup containing two semiconductor lasers, which independently oscillates laser beams having different wavelengths, has been contrived and has been made practicable. However, a size of the pickup resulted in large since two semiconductor lasers are contained in one pickup, and a cost of the pickup is risen since the number of components is increased.
In order to solve such problems, a semiconductor laser in which two emitting regions, each of which emits the light having a different wavelength, are aligned in a transverse direction in one chip as shown in FIG. 13 has been made practicable in TECHNICAL REPORT OF IEICE. CPM99-84 (1999-09). Or, a semiconductor laser element in which two emitting regions, each emitting the light having a different wavelengths, are aligned in a transverse direction in one chip as shown in FIG. 14 has been developed in 47th APPLIED PHYSICS-RELATED ASSOCIATION CONFERENCE, Abstract (29a-N-4, 2000, 3).
In the case where the aforementioned semiconductor laser elements as shown in FIGS. 13 and 14 are utilized for an optical pickup, a system in which the laser beam is divided into three beams by grating, and an information recorded in the disc is read and a tracking position is detected by the divided beam is generally used in the infrared semiconductor laser. There are two side beams on both sides of a main beam from the semiconductor laser element in such three-beams system, and these beams reflected on the disc are returned to the semiconductor laser element. However, in the case where these side beams are returned to an end surface of a laser chip, a problem is arisen that an error signal is emerged in a tracking signal and, thereby, tracking is dysfunctioned, since these side beams are reflected on the end surface of the laser chip and are returned to an optical system again.
As a countermeasure for preventing a tracking error due to the light returning to the end surface of the laser chip as described above, for example, there is contrived in JPA 24030/1986 a method for thinning a thickness of a semiconductor laser element. However, since the distance between the main beam and the side beam is generally 50-80 xcexcm, it is necessary that a thickness of the semiconductor laser element is not larger than the aforementoned distance in order to prevent the light from returning to the end surface of the laser chip by this method. In the case where such a thin semiconductor laser element is manufactured in this manner, there was a problem that the semiconductor laser element can not be stably manufactured since a wafer is frequently split off in a wafering step.
In addition, as another countermeasure for preventing the tracking error due to the light returning to the end surface of the laser chip as described above, for example, there is contrived in JPA 52737/1987 a method in which an emitting point of the laser chip is positioned at the center of the chip by forming a thick film layer on the emitting region of the semiconductor laser element, and this method is made practicable. However, in the case where this method is applied to a horizontal two-wavelengths laser, it becomes nessesarry to form the thick film layer also above the emitting region of the red semiconductor laser. As the result, there was a problem that the red semiconductor laser having the property or reliability at a practical level can not be obtained due to effects such as of a hot step in a thick film layer-forming process and of an increase in the thermal resistance by the thick film layer itself.